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Exploration for Platinum-Group Minerals in Till: a New Approach to the Recovery, Counting, Mineral Identification and Chemical Characterization

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Exploration for Platinum-Group Minerals in Till: a New Approach to the Recovery, Counting, Mineral Identification and Chemical Characterization minerals Article Exploration for Platinum-Group Minerals in Till: A New Approach to the Recovery, Counting, Mineral Identification and Chemical Characterization Sheida Makvandi *, Philippe Pagé, Jonathan Tremblay and Réjean Girard IOS Services Géoscientifiques Inc., 1319 Boulevard Saint-Paul, Chicoutimi, QC G7J 3Y2, Canada; [email protected] (P.P.); [email protected] (J.T.); [email protected] (R.G.) * Correspondence: [email protected]; Tel.: +1-418-698-4498 Abstract: The discovery of new mineral deposits contributes to the sustainable mineral industrial development, which is essential to satisfy global resource demands. The exploration for new min- eral resources is challenging in Canada since its vast lands are mostly covered by a thick layer of Quaternary sediments that obscure bedrock geology. In the course of the recent decades, indicator minerals recovered from till heavy mineral concentrates have been effectively used to prospect for a broad range of mineral deposits including diamond, gold, and base metals. However, these methods traditionally focus on (visual) investigation of the 0.25–2.0 mm grain-size fraction of unconsolidated sediments, whilst our observations emphasize on higher abundance, or sometimes unique occurrence of precious metal (Au, Ag, and platinum-group elements) minerals in the finer-grained fractions (<0.25 mm). This study aims to present the advantages of applying a mineral detection routine initially developed for gold grains counting and characterization, to platinum-group minerals in Citation: Makvandi, S.; Pagé, P.; <50 µm till heavy mineral concentrates. This technique, which uses an automated scanning electron Tremblay, J.; Girard, R. Exploration microscopy (SEM) equipped with an energy dispersive spectrometer, can provide quantitative min- for Platinum-Group Minerals in Till: A New Approach to the Recovery, eralogical and semi-quantitative chemical data of heavy minerals of interest, simultaneously. This Counting, Mineral Identification and work presents the mineralogical and chemical characteristics, the grain size distribution, and the Chemical Characterization. Minerals surface textures of 2664 discrete platinum-group mineral grains recovered from the processing of 2021, 11, 264. https://doi.org/ 5194 glacial sediment samples collected from different zones in the Canadian Shield (mostly Quebec 10.3390/min11030264 and Ontario provinces). Fifty-eight different platinum-group mineral species have been identified to date, among which sperrylite (PtAs2) is by far the most abundant (n = 1488; 55.86%). Textural and Academic Editors: Derek H. mineral-chemical data suggest that detrital platinum-group minerals in the studied samples have C. Wilton and Gary Thompson been derived, at least in part, from Au-rich ore systems. Received: 4 February 2021 Keywords: platinum-group elements; platinum-group mineral; glacial sediments; indicator minerals; Accepted: 27 February 2021 heavy mineral concentrate; automated SEM-EDS mineral analysis Published: 4 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- iations. Despite historically high mineral exploration spending over the past decades, there has been no corresponding increase in new discoveries [1]. The industry is facing a chal- lenge in regard of resource renewal, and new exploration methods are needed, particularly for targeting ore deposits buried under glacial sediment and areas of transported cover that does not respond properly to geophysics, such as in vast areas of the subarctic zone Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. (e.g., Alaska, Canada, Scandinavia, Siberia) [2]. In such environment, the glacial sediments, This article is an open access article although perceived as a hindrance to exploration, led to development of methods capable distributed under the terms and of detecting the signal from the mineral deposits as they are eroded and dispersed in the conditions of the Creative Commons sediments. Till geochemistry and indicator mineral methods have therefore been instru- Attribution (CC BY) license (https:// mental in discovering underlying mineral resources [3]. Dominantly used for gold [4] creativecommons.org/licenses/by/ and diamond exploration [5–7], the technique of using the dispersion of minerals consid- 4.0/). Minerals 2021, 11, 264. https://doi.org/10.3390/min11030264 https://www.mdpi.com/journal/minerals Minerals 2021, 11, 264 2 of 28 ered as footprints of metallogenic environment has recently expanded interest to other commodities including Ni-Cu-platinum-group element deposits [8–11]. Platinum, Pd, Rh, Ir, Ru, and Os, known as platinum-group elements (PGE), are used in a wide variety of industrial applications (vehicles catalysts, fertilizer synthesis, etc.) that are considered as strategic and critical worldwide [12]. Still, approximately 90 percent of PGE originates from South Africa and Russia and they are barely mined out in other industrialized countries [13]. This issue mostly relates to the difficulty of prospecting for this group of metals, notably due to their low abundance, mineralogical complexities, and lack of geophysical or geochemical signatures outside of nickel sulfide occurrences. Hence, the current work aims at presenting the results obtained over the course of the last five years of processing glacial sediment samples using an automated mineral sorting method capable to detect, analyze, and classify platinum-group minerals (PGM) from glacial sediments, and present the usefulness of this new approach for the exploration of PGE-bearing deposits. 1.1. Platinum-Group Elements (PGE) and Their Distribution Platinum group elements have similar physical and chemical properties and occur closely related in nature. These metals are of the rarest elements occurring in the bulk Earth crust, and due to their highly siderophile nature, are mainly concentrated in the dense metallic core [14]. The PGE belong to Group VIII transition metals in the periodic table of elements with Fe, Co, and Ni. Thus, in natural systems they have the ability to be engaged in both metal-metal and covalent bonding to form solid solution with one another, or compounds with siderophiles (e.g., Fe), chalcogenides (e.g., S, Se, Te), or semi-metals (e.g., As, Sb, Te) [8,15]. In nature, these metals form PGE-dominated complexes and alloys, which are known as the platinum-group minerals (PGM), which constitute a very diverse group of minerals (>140 named discrete PGMs) [16]. In magmatic settings, in presence of sulfur or semi-metals (such as As) and due to instability of metallic Fe, the PGE behave as highly chalcophile instead of siderophile elements [16]. Thus, the PGE have high partition coefficients for sulfides in sulfur-saturated magmatic systems [17,18]. Hence, about 99% of the global PGE resources are hosted in magmatic sulfide deposits [19]. Thus, although geochemical characteristics of the PGE allow them to be involved in a great variety of geological processes, our insight into their distribution, and PGMs formation and diversity have mainly been developed by the study of magmatic systems, mostly large layered mafic-ultramafic intrusions (e.g., Bushveld, Stillwater, and Great Dyke complexes). In these, major PGE ore deposits are found as reef- type deposits [17,20], magmatic breccia type such as the Lac des Iles palladium deposit [20], and conduit-type deposits such as those in the Noril’sk-Talnakh area (Russia), which formed as a result of circulation and eruption of vast volume of mafic magma that formed the Siberian Trap Large Igneous Province [21]. Other magmatic deposit types having their importance for the study of the PGMs diversity and composition come from Uralian- Alaskan-type complexes and ophiolite complexes ([22] and references therein). In contrast to the PGMs associated with magmatic systems, there is a poor under- standing about the PGEs’ distribution and mobility during hydrothermal activities and how PGMs may physiochemically modify under low temperature conditions and/or in the surface secondary environment. This is important because the PGMs concentration in placer deposits principally originates from low temperature degradation of large mafic layered intrusions, ophiolitic and/or Uralian-Alaskan-type complexes [23]. Different studies suggest that PGMs composed of native metals and alloys do not typ- ically undergo phase transitions within temperature and/or pressure ranges of geological significance, and they also tend to be relatively resistant to geochemical processes during which lithophile elements can be fractionated and modified in silicate and non-silicate rocks [24–26]. Nevertheless, Hanley [27] discussed the alteration of PGM assemblages precipitated from high temperature liquids at lower temperatures during metamorphism, hydrothermal processes, and/or surficial weathering. Minerals 2021, 11, 264 3 of 28 1.2. Exploration for Platinum-Group Minerals For a long time, placer deposits were the only PGM sources, but currently their contribution to the global resources is less than 4% [28]. The discovery of some primary PGE deposits such as the Merensky Reef of the Bushveld Complex (South Africa), the JM Reef of the Stillwater Complex (MT, USA), the Main Sulfide Zone of the Great Dyke (Zimbabwe), the Noril’sk-Talnakh deposits (Russia), the Sudbury Igneous Complex (Canada), and the Lac des Iles Complex (Canada) has decreased the role of PGM placers as single PGE sources. However, the study of PGM
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